Stroke and vascular dementia are important disabling disorders among the veteran population. Prevention of stroke has the potential to make a great impact, but a vast majority of stroke studies have focused on processes that occur after stroke. To identify molecular process which lead to stroke, we study the prototype human disorder Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy (CADASIL), a hereditary stroke/dementia disorder caused by mutations in Notch3. Hallmark pathological features of CADASIL include: rarefication of arterial extracellular matrix, arterial granular osmiophilic material (GOM) deposition, and vascular Notch3 protein accumulation. The objective of this study is to define molecular processes that cause these abnormalities. Preliminary data demonstrates specific physical and functional interactions between Notch3 and LRP1, a protein known for its endocytic function. We will test the hypothesis that in CADASIL, mutant Notch3 in cerebral vessels impairs LRP1 endocytosis of Notch3, leading to Notch3 buildup and increased MMP expression. We suggest the following: mutant Notch3 in CADASIL dysfunctionally binds to LRP1, leading to LRP1 malfunction; decreased LRP1 results in inhibition of endocytosis of critical extracellular proteins, including Notch3 and MMPs. We propose to test this hypothetic cascade of events in three specific aims. First, we will determine at the molecular level whether mutant Notch3 proteins interact differently with LRP1 (compared to WT Notch3). Second, we will determine in cell cultures whether mutant Notch3 inhibits LRP1 dependent endocytosis. Third, we will quantitate Notch3 buildup in transgenic mice lacking key genes that participate in LRP1 endocytosis to test whether LRP1 is a true target of mutant Notch3 in vivo. These studies may shed light on important targets for prevention of stroke and vascular dementia. A vast literature already support a role for LRP1 and MMPs after stroke. These studies may identify an additional role for LRP1 and MMPs in vascular pathology prior to stroke and perhaps suggest novel methods for stroke and vascular dementia prevention therapy.